The present invention relates to a multilayered electronic device which is useful as a stress sensor element or memory device.
It is well known that certain composite oxides consisting of the oxides of lead, zirconium and titanium as the principal constituents having a perovskite structure, referred to as a lead zirconate titanate (PZT) perovskite material hereinafter, exhibit, when oriented, excellent piezoelectric and dielectric properties so that development works are now under way for the utilization of such PZT perovskite materials as a variety of electronic devices such as stress sensors and memory devices.
Such an electronic device is prepared usually by forming a layer of the PZT perovskite compound on the surface of a single-crystalline substrate having a platinum electrode layer deposited thereon beforehand by sputtering of the component compounds necessary to form the PZT perovskite compound. It is a difficult matter to form an oriented film on a polycrystalline substrate. Since a plurality of components must be involved in the target materials for this sputtering process, a difficulty is encountered in obtaining a fully oriented film of the perovskite compound having a pseudocubic crystalline structure of which the lengths of the a-axis and c-axis are close each to the other, especially, on a polycrystalline substrate. In addition, lead as one of the principal constituents of the PZT perovskite material is liable to cause sublimation and has corrosiveness so that long-term stability can hardly be ensured in a memory device by utilizing the PZT perovskite material due to denaturation of the perovskite material and corrosion on the metallic electrodes.
The present invention accordingly has an object to provide a novel multilayered electronic device having long-term stability of which the working element has a multilayered structure of a PZT perovskite as formed with orientation on a substrate material which is not particularly limitative including single crystal materials as well as polycrystalline ceramic materials and amorphous materials and also to provide a method for the preparation of such a multilayered electronic device.
Thus, the multilayered electronic device of the present invention is an integral body which comprises a three-layered structure consisting of layers (a), (b) and (c), a vertical cross-section of which is depicted in FIG. 8, said layers being:
(a) a substrate of a single-crystalline material, ceramic material, metallic material or amorphous material;
(b) an electroconductive layer formed on the surface of the substrate having a crystalline structure, preferably, of a perovskite of lanthanum nickel oxide LaNiO3; and
(c) a dielectric layer formed on the electroconductive layer having an oriented crystalline structure of a perovskite of, preferably, lead zirconate titanate PZT.
The multilayered structure formed on the surface of a substrate can be an alternate repetition of a multiplicity of the electroconductive LaNiO3 perovskite layers and a multiplicity of the dielectric. PZT perovskite layers.
The above defined multilayered electronic device can be prepared by a method which comprises the steps of:
(A) forming the layer of LaNiO3 on the surface of the substrate by
(A1) coating the surface of the substrate with an aqueous coating solution containing a water-soluble thermally decomposable lanthanum compound and a water-soluble thermally decomposable nickel compound to form a coating layer, and
(A2) subjecting the coating layer formed in step (A1) to a heat treatment in an oxidizing atmosphere to convert the lanthanum and nickel compounds into an electroconductive layer of a LaNiO3 perovskite;
(B) forming a dielectric layer consisting of PZT on the electroconductive layer to form a multilayered structure; and
(C) subjecting the multilayered structure formed in step (B) to a heat treatment at a temperature in the range from 300 to 800xc2x0 C. in an inert atmosphere to convert the layer into an oriented PZT perovskite layer.
The above described step (A) for the formation of the electroconductive LaNiO3 layer and the steps (B) and (C) for the formation of a dielectric oriented PZT perovskite layer can be repeated alternately as many times as desired to prepare a multilayered electronic material having alternate repetition of the electroconductive and dielectric layers.